I think I agree with Platinum fan here. In my opinion, Karin going back to Sasuke was one of the consequences of Kishi's disorganization and urgency to finish the story. It did seem like he had special plans with Karin and maybe even Sasuke himself. But in the end, after pushing Sasuke to decide to join the heroes' effort against Madara, I think he scrapped whatever he was planning and reassembled Taka for the war. It seems pretty clear to me that he had built up Sasuke vs. Karin mostly for Sasuke's sake, to build him up into more of a villain, and probably to give Karin a different storyline in Konoha. What I'm wondering is why he wanted Taka together again at the battlefield. My guess is that he wanted everyone important to the story to be a part of the end, since he no longer had special plans for Karin. Hence bringing her to the front line and having that Naruto/Sasuke - Sakura/Karin scene.
I also think Kishi's just effing with everyone with all the ridiculous love triangles all over the place just to keep things "interesting." So we probably shouldn't take any of this too seriously.